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Keep each API's content type order when negotiating */* (#3032)

A browser's Accept header ends in */*, and on an API whose content
types it names none of, Rack's best_q_match answers it with the first
media type the API declares: the declaration order is the API's
preference. ContentTypes::MimeTypesCache, shared process-wide so that
registries with the same content types share one table, was keyed on the
content-types Hash, and Hash equality ignores order. The first API built
with a set of content types fixed the order for every later API that
declared the same set in another order, so `content_type :json` then
`:md` could answer a browser in Markdown. mime_types_for's shortcut to
MIME_TYPES compared with == and had the same blind spot.

The cache is now keyed on the registry's pairs, in order, and the
shortcut takes DEFAULTS itself only. mime_types_for runs once per
middleware build and is memoized there, so the Array costs nothing per
request.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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